1. Field of the Invention
The present invention relates to a communication system including an interrogator that transmits a carrier wave and a transponder that receives the carrier wave, modulates the carrier wave, and returns the modulated carrier wave as a reflected wave to the interrogator, and also relates to each of the interrogator and the transponder for use in the communication system.
2. Discussion of Related Art
Japanese Patent Publication No. 2000-49656 or its corresponding U.S. Pat. No. 6,177,861 discloses a wireless communication system including a plurality of transponders and an interrogator that sends a carrier wave toward the transponders so that each of the transponders modulates the carrier wave using an information signal such as an ID signal identifying the each transponder or using a subcarrier wave that has been modulated using the information signal, and returns the modulated carrier wave as a reflected wave to the interrogator. If the communication system employs a plurality of interrogators, the interrogators are adapted to transmit respective carrier waves having different frequencies so as to prevent the carrier waves from interfering with each other.
However, there has been a problem that within a signal-reception frequency band of each interrogator, a strong wave transmitted from another interrogator may be mixed with signals transmitted from transponders and mask the same, thereby increasing errors. In particular, in the case where subcarrier waves are used, a wide signal-reception frequency band is advantageous because it can simultaneously deal with the signals sent from the many transponders. In the latter case, however, the wide signal-reception band is more easily interfered with by the other interrogators. In a particular case where a number of interrogators are set on a number of desks in an office, respectively, such that the interrogators are near to each other, noise waves having various frequencies are generated because of mixing and/or intermodulation of the carrier waves, even if the frequencies of the carrier waves differ from each other. Thus, the carrier waves interfere with each other, and the interrogators as a whole consume the increased amount of electric power. In addition, since each transponder does not have the function of selecting frequencies, the transponder modulates and reflects all the carrier waves sent from the interrogators having the different frequencies. Thus, though the interrogators transmit the carrier waves having the different frequencies, each of the interrogators may receive the modulated waves reflected by all the transponders (including other transponders than a target transponder) that are near to the each interrogator.